AIS and a trailing "ghost vessel" -- Solved! (re: Raymarine, Standard Horizon)

[N.A.]

Posting for anyone else who runs into this:

Problem:

My AIS has had a "ghost vessel" on the chartplotter since it was all installed.
[Raymarine Axiom II Pro; Standard Horizon VHF with AIS receive; Vesper 8000 AIS transmit/receive.]

This "ghost vessel" has trailed the boat like a dinghy on the plotter, trailing further behind as boat speed increased. The "ghost" has my own MMSI, so obviously this was a set-up problem. The (otherwise reputable) folks who installed it all did not know what the issue was, but also had trouble seeing it since at the dock the ghost vessel is not obvious against the larger icon for "My Boat".

The presence of the ghost made it impossible to use the proximity (CPA) alarm on the AIS offshore, negating most of the value (to me) of the AIS -- the nearby ghost would cause any proximity / CPA alarm to trigger constantly, forcing me to shut it off.

Solution:

It turns out the Standard Horizon VHS receives AIS and... puts it onto the network with the boat's MMSI. It has not way to ID this on the network as 'My Boat'. The Raymarine chartplotter can ID 'My Boat' (e.g. as put out onto the network by the Vesper AIS) and not trigger alarms off it, but has no way to exclude any MMSI's... including my own, and it cannot recognize my own MMSI as 'My Boat'. Thus it plots both. What the Vesper can do internally I have not investigated yet.

Issue: SH did not do the firmware for their radio correctly. Raymarine did not do the software for their chartplotter correctly. Both should have ways to ID your own vessel's AIS. They do not play together well.

Solution: Disconnect the Standard Horizon VHF from the network. Problem solved. I still have it as backup GPS (displays on radio and handset), it will still work for DSC/emergency signal, but if I want it back on the network (e.g. if my other GPS or AIS fail) then I need to reconnect the network plug for it (not difficult).


Anyway, if anyone else has run into this, here is a solution -- took a long time and involvement of a second (much more skilled) person to solve this on my end.

 

[peaman]

Good that you were able to solve the mystery. I have a very similar setup with no such issues: Same Vesper 8000, Standard Horizon GX2400 with AIS receive but with B&G Zeus chart plotter.

The thing that stands out to me is how the distance from the ghost would vary with boat speed. Either AIS receiver should have a reasonably precise location for your boat independent of speed. It seems to me that maybe the ghost is result of some interference or resonance in the VHF/AIS antenna circuit. Do you have an antenna splitter in your wiring? I don't recall offhand exactly how mine is setup, but I do have a splitter.

 

[peaman]

Do you have an antenna splitter in your wiring? I don't recall offhand exactly how mine is setup, but I do have a splitter.

I have Vesper model SP160 powered antenna splitter which connects with VHF, AIS, and AM/FM.

 

[toddster]

I had that issue several years ago, but it has since gone away. It only happened a few times. Not sure I did anything? Oh, many of the instruments have been upgraded since then though.

BTW, Ghost AIS symbols are a fact of life. They trail behind radar contacts because they are only updated every six minutes. Or less often depending on how far away they are. Even weirder when you look at the PredictWind - generated AIS screen. Satellite-based AIS pings might get updated as often as six minutes, but the PW internet-based AIS pings only once an hour. And if you have their "DataHub" it all gets relayed to your chart plotter.

 

[N.A.]

@toddster is correct that the AIS only gets updated periodically (the actual update rate depends on speed of the transmitting vessel*, actually, but in all cases is long enough that one should not use AIS as a radar substitute in close quarters (i.e. within < 500 yards / 1/4 mile; I will personally be using more like 0.5 or 1nm after discovering this (unhappily) in the fog.) And that is the reason I had the trailing AIS ghost -- my vessel doesn't go that fast, so the AIS update rate (from my transmission) wasn't changing -- it does get shorter at higher speeds, as I said -- but the distance (= time * speed : ) was. Hence the larger trailing distance as speed increased.

But my ghost issue is not what Toddster mentions -- he is talking about the imprecise AIS location due to the transmission delays. My issue was two AIS targets being reported tot he Raymarine plotter: 'My Boat', from the Vesper, and my MMSI, from the VHF detecting my Vesper transmission. The issue was not imprecise location, but rather dual plotting from two sources, not recognized as each other, and with one (coming only from AIS via the VHF detection) not given the boat's actual GPS position on the plotter [ @peaman -- that's why the position of your boat on the plotter is more correct -- the plotter is not plotting your received AIS, but rather your GPS position, which it has. Unless it is also getting another MMSI which is yours but it does not recognize as such, as in my case.]

The issue Toddster mentions is effectively due to the AIS transmission rate standard, is very real, matters critically in low visibility and close quarters, and cannot be fixed (short of having radar). My issue was basically a software issue (with both the Standard Horizon and Raymarine plotter SW being at fault), and can be fixed as I described.

PS: I do have an active (Vesper) splitter, but it -- and any splitter -- is not the cause part of either issue I discuss above.

* PPS: Per Wikipedia, "An AIS transceiver sends the following data every 2 to 10 seconds depending on a vessel's speed while underway, and every 3 minutes while a vessel is at anchor." For me, 6 kts * 10 sec = 100 ft. It looked to me like the icon trailed a bit (sometimes maybe 2x) farther than that, though. And if the other vessel is also moving, then you should probably double all that. Also, as Toddster says (and I did not know), Predict Wind and satellite AIS may update far less often. Knowing that now, personally, I would only trust AIS coming from my own VHF receiver, and then only at distance of > 0.5nm or more. The rest is for tracking your friends during a race / company's ship / knowing what might be coming by sometime tonight.

 

[Second Star]

I have not experienced the significant lag in AIS display noted above. I use a Standard Horizon VHF with AIS rx and display that also displays to a SH plotter. Not "networked" with other instruments through a router. My most dynamic situations are British Columbia ferries travelling at 15-20 knots; that would be distances of 1.5 to 2 miles between pings with a 6 minute delay. I have essentially "real time" display of these vessels and have tracked their passage by visual bearings that confirm the display.
If the sending vessel is tx every few seconds why the delay of minutes in rx and display? Is it an issue of onboard networking slowing down the routing of info? I can see a problem relying on commercial position info from shipping monitoring services via satellite that may only update posit hourly for collision avoidance!